The present invention relates to a method of separating air by a low temperature rectification process in which process streams composed of the air to be separated and fractions of the air that have been separated flow between cooling and distillation stages and a portion of one of the process streams, either containing air or a separated fraction of the air is used to refrigerate the process. More particularly, the present invention relates to such an air separation method and apparatus in which the refrigeration is supplied by expanding the refrigerant stream with the performance of work and after fully warming the refrigerant stream, drawing the refrigerant stream at a sub-atmosphere pressure by a blower or the like to discharge the refrigerant stream at and above atmospheric pressure.
Air is separated into its components by a variety of low temperature rectification processes. In all processes, air is compressed, cooled to a temperature suitable for its rectification is (normally at or near the dewpoint of the air) and then is introduced into the distillation stage having one or more distillation columns to separate the air into nitrogen and oxygen rich fractions.
In any type of air separation plant, there is continual heat leakage into the plant and enthalpy differences between the air feed and product streams at the warm end of the plant. Such heat leakage requires refrigeration to be supplied to the air separation plant. Refrigeration is typically supplied by partially cooling a portion of an incoming air stream or partially warming a waste stream, either rich in nitrogen or oxygen. The air stream, waste or product stream is in turn expanded with a performance of work in a machine known a turboexpander.
In order to transmit the work of expansion, the turboexpander can be coupled to an energy dissipative brake or an electrical generator, or a compressor used in the plant.
The refrigeration output of the turboexpander is related to the pressure ratio of the expansion or more specifically, the pressure ratio of the turboexpander inlet pressure and the turboexpander exhaust pressure. In order to increase the refrigeration output of the turboexpander, in some instances, the inlet pressure to the turboexpander is increased using the shaft energy output of the turboexpander to boost the pressure of the gas destined for turboexpansion. As will be discussed, the present invention provides an air separation method and apparatus in which the amount of refrigeration supplied by the turboexpander is increased by decreasing the turboexpander exhaust pressure.